Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format comprising a number of servo sectors 60-6M-1 recorded around the circumference of the disk 2 that define a plurality of servo tracks 4, wherein data tracks are defined relative to the servo tracks (at the same or different radial density). Each servo sector 6i may comprise a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6i may further comprise groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a servo track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
While the disk drive is centerline tracking a data track during write/read operations, there may be repeatable disturbances induced in the servo control system, such as a repeatable runout (RRO) due to an eccentricity of the servo tracks. There may also be a repeatable disturbance due to a “written-in error” of the servo sectors. The RRO and “written-in error” may be considered fixed disturbances that remain substantially constant over the life of the disk drive. There may also be a variable repeatable disturbance affecting the servo control system due, for example, to degradation of the spindle motor over the life of the disk drive, or changes in the operating environment such as external vibrations affecting the disk drive. When the disturbance is repeatable, compensation values may be learned and then applied to the servo control system as each servo sector is processed, thereby reducing the undesirable effect of the repeatable disturbance.